A radar apparatus is disposed in a vehicle and is used as a safety device for collision prevention. As an example of a radar apparatus disposed in a vehicle, a frequency-modulated continuous wave radar apparatus (FWCW radar apparatus) is used. The FWCW radar apparatus can detect both a distance to and a relative speed of a target. The FWCW radar apparatus has a simple configuration and is appropriate for reducing a dimension and a cost.
The FM-CW radar apparatus transmits a transmission signal Ss, which is frequency-modulated with a triangular wave to have a frequency increased and decreased gradually in a linear fashion, and receives a wave reflected by a target as a reception signal Sr.
The reception signal Sr delays for a time Td for which the transmitted and received wave takes to travel from the radar apparatus to the target and back, that is, time depending upon the distance to the target, and the reception signal Sr is Doppler-shifted in frequency by Fd depending upon the relative speed of the radar apparatus and the target.
The reception signal Sr and the transmission signal Ss are mixed together by a mixer to produce a beat signal B, whose frequency is equal to a difference in frequency between the reception signal Sr and the transmission signal Sb. If the frequency of the beat signal B (hereafter, referred to as an upbeat signal Bu) when the frequency of the transmission signal Ss is increased (hereafter, referred to as a beat frequency in a modulated frequency-rising range) is defined as fu, and the frequency of the beat signal B (hereafter, referred to as a downbeat signal Bd) when the frequency of the transmission signal Ss is decreased (hereafter, referred to as a beat frequency in a modulated frequency-falling range) is defined as fd, a distance R to and a relative speed V of the target may be expressed as:
                    R        =                                            c              ·              T                                                      8                ·                Δ                            ⁢                                                          ⁢              F                                ·                      (                          fu              +              fd                        )                                              (        1        )                                V        =                              c                                          4                ·                F                            ⁢                                                          ⁢              0                                ·                      (                          fu              -              fd                        )                                              (        2        )            where c is a propagation speed of a radio wave, T is a period of the triangular wave for modulation of the transmission signal Ss, AF is a variation in frequency of the transmission signal Ss, and Fo is a central frequency of the transmission signal Ss.
In a radar apparatus disposed in a vehicle, it is important to measure the azimuth or angular direction of a target as well as the distance R to and the relative speed V of the target.
Japanese Patent No. 3,622,565 (corresponding to U.S. Pat. No. 6,292,129) discloses a radar apparatus capable of detecting an azimuth of a target. The radar apparatus includes a plurality of receiving antennas and a receiver. The receiver includes a receiver switch, a switching control circuit and a receiving circuit. The receiving antennas are divided into a plurality of receiving group. The receiver switch selectively establishes communication between one of the receiving antennas in each receiving group and the receiving circuit. The switching control circuit controls an operation of the receiver switch so as to change the communications between the receiving antennas and the receiving circuit in sequence in a cycle shorter than a cycle of a change in a frequency of a transmission signal. The receiving circuits mix the reception signals from the receiving antennas in each receiving group with a local signal to generate a reception signal Sr. The radar apparatus acquires azimuth information as well as a distance and a relative speed by pair matching of peaks of an upbeat signal Bu and a downbeat signal Bd output from a mixer and an arrangement of the receiving antennas which are selected at the time.
In a radar apparatus that derives information including a position of a target by pair matching of peaks of an upbeat signal Bu and a downbeat signal using a FMCM method, when a digital signal processing such as fast Fourier transformation (FFT) is carried out, a reception signal of a target that is present at a distance greater than a Nyquist frequency to a sampling frequency of the reception signal may be folded to a position less than the Nyquist frequency, the target at a long distance may appear at a short distance, and erroneous detection may be caused.